A file share with Word and Excel documents (.doc, .docx, .xls, .xlsx) having custom document properties is indexed via MOSS 2007 or MSS 2008.
When the crawl has finished the custom properties are listed in 'Crawled properties' but the details view mentions "There are zero documents in the index using this property."
However if you create a Managed Property from this Crawled Property it does contain the correct values and can be used as desired (query, filter, sort, etc).
Hooray !